1. Field of the Invention
The present invention relates to a non-contact measurement of the wheel alignment of motor vehicles, and more particularly to a method and an apparatus for optoelectronically measuring the vehicle wheel alignment.
2. Prior Art
The wheel alignment is an important requirement for safe and stable travelling of motor vehicles. It is therefore necessary to accurately measure the vehicle wheel alignment at a high speed for enabling proper subsequent adjustment of the wheel alignment based on the result of the measurement.
There are known two types of measurement of vehicle wheel alignment; one is achieved statically and the other dynamically. The dynamic measurement is preferable because it is conducted under conditions substantially the same as the actual vehicle travelling conditions.
According to one such known dynamic measurement system, a pair of axially movable roller-headed feelers or proves is held in contact with a tire side surface at diametrically opposite portions thereof for measuring a toe-in angle and a camber angle on the basis of axial displacements of the respective proves. This system is however disadvantageous in that due to irregularity of the tire side surface, the measurements of a toe-in angle or of a camber angle fluctuate and hence an accurate measurement is difficult to achieve.
Another example of known dynamic measurement system is constructed to detect a side slip force of the tire (i.e., a force urging the tire to slip laterally when a vehicle is running) which varies proportional to the magnitude of toe-in angle of the wheel. This system is however disadvantageous in that the correlation between the side slip force and the toe-in angle is not constant but varies depending on the tread patterns of the individual tires.
As described above, the known dynamic wheel alignment measurement systems are not satisfactory in accuracy because they are negatively affected by the shape, design and deformation of a tire to be measured.
With the foregoing difficulties in view, an artisan might attempt to realize an optical measurement system capable of measuring the wheel alignment without direct contact with a tire. A possible proposal would probably be a system such as shown in FIG. 8 of the accompanying drawings. The system includes a pair of photoelectric sensors S1, S2 disposed near the tire T on a wheel W for measuring the distance between a side surface of the tire T and a reference vertical plane. With this arrangement, however, when the wheel W is rotated by a pair of drive rollers (not shown), it is displaced rearwardly, as indicated by the dotted lines. Relatively to this rearward displacement of the wheel W, measuring points M1, M2 on the tire T are displaced forwardly. With this displacement of the measuring points, the obtained measurements of a toe-in angle .theta. necessarily contain an error.